Liquid crystalline materials have properties that are intermediate those of a true liquid and a true crystal since they have an ordered structure while also having fluidity. These materials are known and are characterized or identified by one of three phases or structures known as the smectic phase, the nematic phase, and the cholesteric phase (a special form of the nematic phase). The present invention is concerned with materials exhibiting a cholesteric liquid crystalline phase.
Compounds with the cholesteric liquid crystalline structure exhibit certain characteristics which are markedly different from those having the smectic or the nematic structures. The characteristic properties of compounds with the cholesteric liquid crystalline structure may be summarized as follows: (1) they are optically negative, in contrast to the smectic and nematic structures which are optically positive; (2) the cholesteric liquid crystalline structure is optically active and shows strong optical rotatory power; (3) when illuminated with white light, the most striking property of the compounds with the cholesteric liquid crystalline structure is that they scatter light selectively to give vivid colors. The color and intensity of the scattered light depends upon the temperature of the scattering material and upon the degree of incidence of illumination. A cholesteric material exhibits a scattering peak having a band width of about 200 angstroms that occurs in or between the infrared and ultraviolet portions of the spectrum; (4) in the cholesteric structure, one circular polar component of the incident beam is completely unaffected. For the dextro cholesteric structure, it is only the circular polarized beam with counterclockwise rotating electric vector which is reflected. (The sign of rotation refers to an observer who looks in the direction of the incident light.) Levo cholesteric structures have the reverse effect; (5) when circular polarized light is scattered from these materials, the sense of polarization is unchanged. In ordinary materials, the sense of circular polarization is reversed; (6) the mean wave length of the reflected band depends upon the angle of incidence of the beam. The relationship can be roughly approximated by the Bragg difraction equation for birefringent materials. These enumerated properties effectively define cholesteric liquid crystals.
Thin films of cholesteric liquid crystals exhibit a property upon interaction with light, which may be termed "selective scattering". The term "scattering" is used rather than "reflection" in order to distinguish from the effect occurring on mirror surfaces wherein light is reflected at an angle equal to the angle of incident light. A scattered light ray may leave the scattering material at an angle unrelated to the angle of the incident light. A selectively scattering film, when observed with light impinging the film on the same side as that which is viewed, has an apparent color which is a complement of the color of the light transmitted through the film.
The terms "light" and "color" as used herein have a broad connotation of referring to electromagnetic radiation generally, rather than to solely visible radiation.
The phenomenon of selective scattering as exhibited by cholesteric liquid crystalline films is independent of whether the light illuminating the film is polarized or not. The color and intensity of the scattered light depends upon the temperature of the scattering material and upon the angle of incidence of illumination.
Because of the thermochromic properties of cholesteric liquid crystals, films containing them are useful for detecting temperature patterns on various objects, i.e., thermography and/or themometry. This temperature pattern is manifested by an irredescent color pattern exhibited by compounds in their cholesteric liquid crystalline phase.
Compounds capable of existing in the cholesteric liquid crystalline phase exhibit thermochromic properties at temperature ranges which are unique for that compound. Therefore, the particular cholesteric liquid crystalline compound or mixtures of compounds utilized to detect a temperature pattern can be varied to result in color sensitivity at the particular temperature range being measured. At the lower temperature within the range, which can be varied from fractions of a degree to several degrees of temperature, the color exhibited is in the red end of the spectrum and at the higher temperature within the range, the color is in the violet end of the spectrum. Intermediate temperatures result in intermediate colors, e.g., green. Thus, for example, if it is desired to measure and detect the temperature pattern of a particular portion of the anatomy of a person suspected of having a blood circulatory disorder or a tumor, a composition which shows a color change at the appropriate temperature can be formulated. Furthermore, cholesteric liquid crystals have been utilized to determine faults of metal parts of machines and airplanes by non-destructive testing techniques.
Previously, it has been found that in order to more easily visualize the colors exhibited by cholesteric liquid crystals, it is advantageous to utilize a black background. However, the use of a black background gives rise to problems which make the use of cholesteric liquid crystals for detecting temperature patterns difficult and uneconomical. One problem is that the black background must be painted on in the form of a paint or a spray and then the liquid crystals must be applied to the black background so the colors can be readily observed. Because of the problems involved, the adaptability of these systems is limited. Further, these methods are disadvantageous since the oily cholesteric liquid crystals must be applied to the black background as a solution in a volatile solvent, thus causing obvious dangers. Furthermore, the removal of the background and particularly the liquid crystals themselves is difficult particularly where large areas are concerned. These methods are also disadvantageous since it is very difficult if not impossible to get a uniformly even coating of the liquid crystals upon the background, thus rendering the pattern unreliable. Furthermore, by the known methods, the re-use of the liquid crystals is, for practical purposes, impossible.
In instances wherein the black background is painted or sprayed on a plastic film prior to the application of liquid crystals, as well as wherein no plastic film is utilized, problems arise since the liquid crystals age and are unstable when exposed to the atmosphere causing partial decomposition of the compounds and loss of color intensity and either a shift in the color-temperature response or complete loss of the color-temperature response. Even in the case wherein the liquid crystals are protected from the atmosphere, e.g., minute transparent liquid walled capsules, aging and other problems arise. This is true since films formed containing these materials tend to be rough and the protected liquid crystalline material can be rubbed off, thus causing losses of color intensity and thermographic reliability. Furthermore, the thin walls of the capsules can shatter under pressure, thus exposing unprotected liquid crystals to the atmosphere.
It is more advantageous to use a film of the cholesteric liquid crystals, preferably on a flexible substrate blackened prior to the application of liquid crystals, since the utilization of a black paint or spray to serve as a background is particularly troublesome when dealing with human patients since these black paints are very difficult to apply as a uniform film, are uncomfortable on a patient and difficult to remove.
There is thus a need for a stable cholesteric liquid crystalline composition that is amenable to re-use, exhibits good color properties at desired temperature ranges, can be formed into or onto a film and is easy to apply as a uniformly thick film, is easy to remove from the thermography subject and permits the use of an easily handled black or dark background.
The problem of the prior art can be overcome to some extent by using film forming emulsions containing cholesteric liquid crystal materials. These emulsions are generally satisfactory for protecting and stabilizing the liquid crystal compositions, however, they do not prevent aging of the liquid crystals during storage.